fix: 新增后端 /api/v1/salary/dashboard 路由

- 创建 salary.routes.ts，提供薪酬分析数据
- 基于 Payslip 模型计算部门对比、月度趋势、同比环比
- 在 app.ts 中挂载 /api/v1/salary 路由

backend/src/app.ts

@@ -65,6 +65,7 @@ import specialStatusRoutes from './routes/special-status.routes'
 import companyFileRoutes from './routes/company-file.routes'
 import acceptanceTestRoutes from './routes/acceptance-test.routes'
 import leaveRoutes from './routes/leave.routes'
+import salaryRoutes from './routes/salary.routes'
 app.use('/api/v1/auth', authRoutes)
 app.use('/api/v1/dashboard', dashboardRoutes)
 app.use('/api/v1/employees', employeeRoutes)
@@ -93,6 +94,7 @@ app.use('/api/v1/special-statuses', specialStatusRoutes)
 app.use('/api/v1/company-files', companyFileRoutes)
 app.use('/api/v1/acceptance-tests', acceptanceTestRoutes)
 app.use('/api/v1/leaves', leaveRoutes)
+app.use('/api/v1/salary', salaryRoutes)
 
 app.use(errorHandler)